1. Field of the Invention
The present invention relates to a power feed line switching circuit for a submarine branching device and a method of feeding power to a submarine cable communication system. More particularly, the present invention is directed to a power feed line switching circuit for a submarine branching unit incorporated into a submarine cable communication system in which submarine cables are branched undersea to enable communication between three or more land stations, and further, to a method of feeding power employing the power feed line switching circuit.
2. Description of the Related Art
Generally, a submarine cable communication system, particularly, a submarine optical communication system using optical marine cables (OMC) is provided with optical repeaters (REP) at specific intervals in each of the optical marine cables. Note, in the prior art, a one-end power feed system (mode) and a both-end power feed system have been used in the submarine optical communication system.
A constant DC current is supplied through a power feed line of the optical marine cable to the optical repeaters. The DC current is supplied to the optical repeaters in either the one-end power feed mode in which the DC current is supplied from only one land station or the both-end power feed mode in which the DC current is supplied from two land stations. The both-end power feed mode is preferable because of a high reliability, and it is preferable to switch the power feed line when a fault occurs in the power feed line so that a set power feed line is a both-end power feed line, as much as possible.
In the one-end power feed system having three cable landing stations of the prior art, respective power feed units of the three cable landing stations are connected individually by power feed lines to a submarine branching unit (BU). Power is fed in the one-end power feed mode only from each of the cable landing stations to the submarine branching unit and the repeaters.
The conventional submarine optical cable communication system including three or more cable landing stations sets a power feed line by changing the power feed line switching circuit of the submarine branching unit for one-end power feed or both-end power feed. Note, the conventional power feed line switching circuits for a submarine optical cable communication system communicating between three or more cable landing stations are, for example, disclosed in Unexamined Japanese Patent Publication (Kokai) Nos. 2-53332, 1-200832, 1-276937 and 63-189025 (corresponding to U.S. Pat. No. 4,798,969 and G.B. Patent No. 2,202,110). The configurations and problems of these prior art power feed line switching circuits will be explained later with reference to the accompanying drawings.